An engine may be operated diluted with EGR to improve engine fuel economy and emissions. The engine may be operated less throttled when EGR flow is increased to engine cylinders; however, combustion stability of the engine may degrade. Engine combustion stability may degrade due to slower combustion, decreased ignitability, variation in the amount of EGR supplied to engine cylinders, and variations in engine operating conditions where the present EGR flow rate was determined. For example, intake air temperature may increase or decrease, ambient humidity may increase or decrease, fuel octane may vary, and the commanded EGR flow rate may not be exactly the EGR flow rate produced. Therefore, it may be desirable to develop a way of providing stable engine combustion in the presence of high levels of dilution even when operating variables may vary.
The inventors herein have recognized the above-mentioned disadvantages of operating a highly diluted engine and have developed an engine operating method, comprising: injecting a lower hydrogen concentration fuel to a group of cylinders; and; increasing a fuel injection amount of a higher hydrogen concentration fuel to one or more dedicated EGR cylinders in response to combustion stability in the group of cylinders being less than a desired combustion stability.
By increasing an amount of higher hydrogen concentration fuel injected into a cylinder that supplies exhaust gas to other engine cylinders, it may be possible to improve engine combustion stability for an engine that operates with a higher level of exhaust gas dilution. For example, an air-fuel ratio of a dedicated EGR cylinder (e.g., a cylinder that directs at least a portion of its exhaust flow, without exhaust from other cylinders, to provide external EGR to engine cylinders) may be richened to an air-fuel ratio that is richer than a stoichiometric air-fuel ratio of the gaseous air-fuel mixture combusted in the dedicated EGR cylinder to produce excess hydrogen in the dedicated EGR cylinder's exhaust gas. The exhaust gas from the dedicated EGR cylinder may be provided to other engine cylinders where the excess hydrogen may improve combustion stability. And, since the fuel supplied to the dedicated EGR cylinder has a higher hydrogen concentration, higher levels of excess hydrogen may be supplied in the exhaust gas as compared to if a same lower hydrogen concentration fuel combusted in the engine's remaining cylinders were combusted in the dedicated EGR cylinder. In this way, it may be possible to operate a highly EGR diluted engine with a desired level of combustion stability.
The present description may provide several advantages. Specifically, the approach may improve combustion stability of a highly EGR diluted engine. Further, the approach may provide a simplified cost effective way of improving engine combustion stability. Further still, the approach may allow an engine to operate more efficiently and stable at light loads and low engine speeds.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.